The invention relates to systems and/or devices for conversion of electrical energy into acoustical energy, eg loudspeakers; and/or for the conversion of acoustical energy into electrical energy, eg microphones.
The basis for the design of many types of moving-coil-dynamic loudspeakers is set forth in a 1927 article entitled, "Notes on the Development of a New Type of Hornless Loudspeaker," by Chester W. Rice and Edward W. Kellogg, published in the AIEE Journal. As therein described, the primary determinents were the use of a field coil, which dictated the shape of the magnetic structure; and, since the best materials then available for diaphragm fabrication were paper and thin spun aluminum, it was necessary to form the diaphragm into a cone shape for structural rigidity. In the just cited article, it is recognized that the ideal diaphragm would be completely rigid and move as a unit (piston action) in response to the forces applied by the voice coil.
Within the Rice-Kellogg design, a larger voice coil diameter would necessitate an increase in the mass of the moving system and a resultant decrease in efficiency which was unacceptable due to the low power from audio amplifiers then extant. For example, the special "high power" amplifier developed by Rice and Kellogg produced only 70 milliampers at 200 volts; and this small output power required that the speaker voice coil diameter be small, eg 1.00" to 2.00", by comparison to the 3.00" to 4.125" voice coil diameter used in high power loudspeakers of current design. Loudspeaker acoustic output requirements have increased as high sound pressure levels are often required, for example, in applications involving large auditoriums and/or "rock-level" sound volumes; and there are now many amplifiers capable of generating power outputs in excess of 200 watts at 8 ohms.
Electromagnets as used in early loudspeakers utilized a metal core which strengthened the magnetic field generated by the field coil current by a factor of about 1,000 over the magnetic field strength generated with an air core. In these designs the voice coil windings were short, eg 1/4", and the windings were enclosed by the static magnetic flux throughout the entire range of axial excursion, yielding a comparatively high efficiency. Because of the limited axial displacement, diaphragm size was maximized and/or horn loading was utilized.
When hard steel metal magnets replaced field coil designs (circa 1931 Jensen Manufacturing Company), magnets with large volumes between the working pole faces were required to produce acceptable efficiency. The magnetic return structure, a low reluctance soft iron material equivalent to the electro-magnetic metal core, was retained. With the subsequent introduction of more advanced materials such as Alinco 5 types, barium ferrite (ceramics), and rare earth cobalt micropowders, advances in stability and long term performance were made; eg, reduced sensitivity to magnetic loss due to heat and shock was achieved.
However, current art magnetic structures suffer from losses of the actual energy product B (flux density).times.H (coercivity) of the magnet via:
1. leakage--which occurs at all joints between the pieces of the structure;
2. fringing--due to spurious magnetic circuits other than the working air gap; and
3. hot spots--uneven distribution of flux density within the return structure.
U.S. Pat Nos. 2,141,208, 2,548,235, and 2,756,281 illustrate various configurations of magnetic structures used in prior art loudspeakers.
Other loudspeaker developments include the use of advanced materials for the cone diaphragm; plastics; polystyrene, polypropylene, expanded polystyrene foam, metals; aluminum, beryllium, boron, and plastic-metal combinations, such as expanded polystyrene foam laminated with aluminum foil and honeycombed aluminum with a polystyrene laminate. In addition, formulations of paper reinforced with stiff carbon fibers have also been utilized in various designs to increase rigidity and reduce cone breakup which is an important contributor to distortion in loudspeakers. In other designs, various materials have improved (controlled the mechanical resistance-compliance of) the voice coil centering spider and cone surround which, as Rice and Kellogg recognized, were important design goals.
However, in todays most widely used speakers, the basic mechanical shape and parameters have not significantly changed from those described by Rice and Kellogg. The dynamic loudspeaker is most often found to be a cone (convex-funnel) shaped diaphragm driven at its apex by a moving voice coil situated in a permanent magnet field. However, U.S. Pat. Nos. 2,655,566 and 2,756,281 disclose configurations whereby a centrally disposed rod drives a conically shaped diaphragm from its apex, and the rod is driven by a voice coil assembly. U.S. Pat. No. 2,069,242 purports to be a vibratory system of true piston or plunger action, and employs a driven member which takes the form of a truncated cone of rigid construction which is driven by a flexing cylinder which may be flexed in an accordian fashion.
The acoustic output of a loudspeaker is a function of diaphragm size and diaphragm displacement (excursion capability); other variables include electromagnetic conversion efficiency, rigidity of the diaphragm, and the acoustic impedance and capacitance of the air to vibration at various frequencies. Previous attempts to produce greater levels of acoustic output and deeper bass reproduction have generally been limited to increasing the diaphragm diameter of the loudspeaker, because, leaving aside the current art magnetic design limitations discussed above, the surround and spider (suspension elements) of current design speakers typically limit useful cone excursion to an average upper limit of .+-.0.25 inch (one-half inch peak-to-peak). This excursion limit places a limit upon both the frequency (low frequency) which can be produced and the sound pressure level of the reproduction. For example, in order for an 8" diameter loudspeaker to produce one acoustic watt at thirty hertz (cycles) it must undergo a .+-.2.0" excursion. This acoustic output is clearly outside the capability of the present designs which, as stated above, have a usable excursion range of about .+-.0.25 inch. Other current art designs have voice coil windings which are longer than the magnetic return plates which define the linear area of the magnetic field and therefore have reduced efficiency.
In most types of current design speakers, the centering spider and surround mechanically inhibit cone vibration at some frequencies, and cause a run-on of vibration at other frequencies. Generally there is a characteristic run-on of vibration in the lower output range which is called the bass resonant frequency (BRF) of the loudspeaker. The BRF is affected by electromagnetic factors such as amplifier output dampening; the magnetic flux strength in the voice coil gap; the acoustic loading factors such as the type and size of the enclosure or the placement of the enclosure in the listening environment. The BRF is also affected by the mechanical aspects of the loudspeaker, such as the mass of the diaphragm, or the axial compliance of the suspension elements. Any of these factors can contribute to a frequency response, and output pressure level disproportionate to the relative level of the input signal. Hence, in loudspeakers of current designs, surround and spider contribute to amplitude distortion, and large low frequency excursions are compressed by the suspension limits. Also the suspension elements contribute to amplitude distortion due to their frequency dependent mechanical resistance to axial travel.
As Rice and Kellogg described it, the ideal design would be inertia controlled by the mass of the moving elements and would be operated above the lower natural resonant frequency; rather than resistance controlled where the major resonant frequencies occur within the intended range of reproduction. Rice and Kellogg showed that a completely rigid inertia controlled diaphragm, when driven by a constant amplitude electrical signal, would have a linear acoustic output over its working range.
There is no completely rigid material or design shape, and in the current design speakers, depending upon the thickness, density, configuration and materials used, the diaphragm does not act as a rigid piston. Vibrations originating in the motor elements are driven through the diaphragm from apex to surround at various rates, and depending upon the diaphragm design, various frequencies are absorbed or reinforced due to antiphase movements of the diaphragm itself. Because the non-rigid diaphragm acts as a transmission line for vibration, and because the speed of sound in the surrounding air and the cone material often differ, significant amplitude and frequency modulation distortion occurs.
Regarding diaphragm and voice coil former movement, ideally there is high axial flexibility and no flexibility at any angle which would allow the voice coil former to strike or rub against the magnetic assembly. As a result, in current design high output speakers, a comparatively large magnetic gap is needed, as high excursions often result in skewed motor movement and a distortion of the circular shape of the voice coil former at the apex of the diaphragm. However, a large magnetic gap reduces the magnetic flux density in the gap which may exaggerate the bass resonant frequency peak, increases the response time to input signals (lower transient response), and increases the working temperature of the voice coil wire--which increases the resistance of the wire, lowers the electromagnetic efficiency, and contributes to voice coil burnout.